Underreporting of Side Effects to VAERS

Still, although reports to VAERS are underreported, they are almost certainly not underreported by as much as some folks believe.

Have you heard the claim that only 1% of serious vaccine reactions are reported to VAERS?

That’s not true.

That claim is based on an old study about drug reactions and was not specific to vaccines.

We also know that underreporting is less common for more severe adverse reactions than for those that are more mild. For example, one study found that up to 68% of cases of vaccine-associated poliomyelitis (a table injury) were reported to VAERS, while less than 1% of episodes of rash following the MMR vaccine were reported.

That’s not to say that only severe or serious adverse reactions should be reported.

But since VAERS watches “for unexpected or unusual patterns in adverse event reports,” it still works even if each and every side effect isn’t reported.

VAERS Works

Reports to VAERS are underreported.

VAERS still works well though.

Again, that’s because VAERS doesn’t need each and every adverse event to be reported for the system to work and to help it identify vaccines that might not be safe.

“Despite its limitations, VAERS effectively detected a possible problem soon after introduction of RRV-TV in the United States.”

Lynn R. Zanardi, et al on Intussusception Among Recipients of Rotavirus Vaccine: Reports to the Vaccine Adverse Event Reporting System

We saw that with the RotaShield vaccine. After nearly 20 years of research, the first rotavirus vaccine was approved by the FDA on August 31, 1998. About seven months later, in March 1999, the ACIP published their formal recommendations that all infants get RotaShield on a three dose schedule, when they are two, four, and six months old.

By June 11, 1999, VAERS had received 12 reports of intussusception related to the RotaShield vaccine and by July 13, its use was temporarily suspended, as the CDC continued to investigate.

Once the CDC announced the temporary suspension, even more reports of intussusception after RotaShield were made to VAERS. Those extra reports likely mean that intussusception was being underreported initially, but it still triggered the temporary suspension and extra studies that eventually got the manufacturer to withdraw the vaccine.

“VAERS is used to detect possible safety problems – called “signals” – that may be related to vaccination. If a vaccine safety signal is identified through VAERS, scientists may conduct further studies to find out if the signal represents an actual risk.”

CDC on How VAERS is Used

Early signals in VAERS also helped detect a very small increase in the risk of febrile seizures among toddlers who got the 2010-11 flu vaccine combined with either Prevnar or a DTaP vaccine.